Process for recovering aliphatic dibasic acids



United. States Patent "ice PROCESS FOR RECOVERING ALIPHATIC DIBASIC ACIDS Russell G. Hay, Fox Chapel, and Stanley M. Hazen, Cheswick, Pa., assignors to Gulf Research & Developrneut Company, Pittsburgh, Pa., a corporation of Delawere No Drawing. Application .lune 14, 1955,

Serial No. 515,517

16 Claims. (11. 260-537 This invention relates to a method for removing dibasic carboxylic acids having an even number of carbon atoms from a mixture of which said dibasic carboxylic acids form a part.

Dibasic carboxylic acids having an even number of carbon atoms are often found in admixture with other compounds or compositions from which it is desired to separate them. They may be found for example in admixture with dibasic carboxylic acids having an odd number of carbon atoms or nitric acid or both. A convenient and effective method for removing dibasic carboxylic acids having an even number of carbon atoms, such as adipic acid, from mixtures containing such acids is of course desirable.

The problem of separating dibasic carboxylic acids from mixtures or solutions containing the same is often encountered with the products obtained as a result of oxidation, such as nitric acid oxidation, of hydrocarbons such as saturated aliphatic hydrocarbons, cycloparaffins, cyclic ketones, etc. Not only are dibasic carboxylic acids having an even number of carbon atoms to be found in the products obtained in such processes, but often dibasic carboxylic acids having an odd number of carbon atoms, monobasic acids, unreacted nitric acid, etc., are also present. The recovery and removal of dibasic carboxylic acids having an even number of carbon atoms from these oxidation processes is a diificult problem. As an example, but without intending to be limited thereby, the process of this invention can be employed to separate dibasic carboxylic acids having an even number of carbon atoms from oxidation products obtained in accordance with such processes as those disclosed and claimed in applications Serial Numbers 376,616, new U. S. Patent No. 2,771,482, and 376,664 of Brown et al., both filed August 26, 1953, alsoassigned to the assignee of the present invention. Briefly, dibasic carboxylic acids are produced in accordance with the processes in the applications by air oxidizing a saturated aliphatic hydrocarbon and thereafter subjecting at least a portion of the airoxidation product obtained to further oxidation with nitric acid under conditions effective to obtain substantial amounts of dibasic carboxylic acids.

We have found that dibasic carboxylic acids having an even number of carbon atoms can be removed from a mixture or solution of which said dibasic carboxylic acids form a part by aprocess which comprises admixing a nitrohydrocarbon with said mixture, holding the re sulting mixture at a temperature at which substantially all of said dibasic carboxylic acids are precipitated, and thereafter removing said precipitated acids from said resulting mixture.

The process of this invention can be employed to remove any dibasic carboxylic acid having an even number of carbon atoms from a mixture or solution of which said dibasic carboxylic acid forms a part. While good results are obtained in accordance with this invention when the dibasic carboxylic acids to be removed have an even num- 2,804,475 Patented Aug. 27, 1957 her of carbon atoms of two to about 20 carbon atoms per molecule, especially good results are obtained when such dibasic carboxylic acids to be removed have an even number of carbon atoms of two to about 10 carbon atoms per molecule.

In carrying out the process of this invention any nitrohydrocarbon can be employed which will selectively dissolve substantially completely all of the ingredients forming a part of the solution or mixture containing the dibasic carboxylic acids having an even number of carbon atoms but not the said dibasic carboxylic acids. The nitrohydrocarbon can thus be straight chain, branched chain or closed chain. Of the nitrohydrocarbons which can be employed with greatest advantage are those having from one to 6 carbon atoms per molecule, particularly those having from one to 3 carbon atoms per molecule. Specific nitrohydrocarbons which can be mentioned as being representative are nitromethane, nitroethane, 1,1- dinitroethane, nitropropane-l, nitropropane-Z, nitrobutane-l, nitrobutane-Z, nitropentane-l, nitropentane-Z, 2- methyl-4-nitrobutane, secondary S-nitropentane, nitrohexane-l, nitrohexane-2, nitrocyclohexane, nitrobenzene, etc.

The amount of nitrohydrocarbon which must be employed is not critical and can be varied over a wide range provided it is sufficient to selectively dissolve substantially completely all of the ingredients forming a part of the mixture or solution being treated except the dibasic carboxylic acids having an even number of carbon atoms. For example, if the mixture being treated contains dibasic acids having an even number of carbon atoms and nitric acid, the amount of nitrohydrocarbon which must be used is that necessary to dissolve the nitric acid. If, in addition, the mixture also contains dibasic acids having an odd number of carbon atoms, the amount of nitrohydrocarbon which must be used is that necessary to dissolve not only the nitric acid but also the dibasic carboxylic acid having an odd number of carbon atoms. In general We prefer to employ the nitrohydrocarbon in an amount in excess of'that required to dissolve all of the ingredients of the mixture being treated except the dibasic carboxylic acids having an even number of carbon atoms. The employment of at least two volumes, and preferably 3 to 5 volumes of solvent per volume of ingredient to be dissolved is satisfactory for purposes of the invention.

After the nitrohydrocarbon has been added to the mixture containing the dibasic carboxylic acids having an even number of carbon atoms, the resulting mixture is maintained at a temperature and for a time sufiicient to precipitate said dibasic carboxylic acids. The temperature of precipitation is not critical and can be varied over a great range. In part it depends on a number of variables, as for example thetype and nature of the mixture being treated, the particular nitrohydrocarbon and the amount being used, the rate at which it is desired to cause precipitation to occur, etc. In general a temperature of about 0 to about 100 F. can be employed, and a temperature of about 32 to about F. is preferred. The time for precipitation to occur and cease depends in part on the same variables set forth above in connection with the nitrohydrocarbon employed. The time for precipitation may be as low as about 5 minutes, but generally varies from about one to about 24 hours.

After precipitation has ceased and the bibasic carboxylic acids having an even number of carbon atoms have come out of solution, they are removed from the solution by any convenient and effective method such as filtration, decantation, etc. Preferably the crystals of dibasic carboxylic acids so removed are thereafter washed with a nitrohydrocarbon, generally the same as that employed in the separation step, in order to remove thereand it is therefore the only one removed in the process,

no further treatment may be necessary and it may be used as such immediately. If two or more dibasic carboxylic acids having an even number of carbon atoms are present and are removed from the mixture or solution containing them, further treatment to separate one dibasic carboxylic acid having an even number of carbon atoms from another dibasic carboxylic acid having an even number of carbon atoms may be desirable- Separation in the latter instance can be effected, for example, by esterifying the mixture of dibasic acids having an even, but different, number of carbon atoms with an alcohol such as methyl alcohol and thereafter distilling the product to obtain fractions of esters. Each of these fractions can then be hydrolyzed in a pressure reactor with water and a catalyst, freeing the dibasic acid and alcohol. The alcohol is distilled off, leaving a mixture containing the dibasic acid and water. If the dibasic acid is soluble in the water, the water is removed therefrom by distillation; if the dibasic acid is insoluble in the water, the water can be removed by ordinary separation procedures such as filtration or decantation.

A method of removing dibasic acids having an even number of carbon atoms from a mixture obtained from the oxidation of a paraflin hydrocarbon is illustrated below in Example I.

Example 1 110.2 grams of a 132 F. melting point refined paraffin wax is placed in a flask equipped with a stirrer and a condenser. The wax is heated to a temperature of 160 C. and maintained at that temperature while air is blown therethrough. Throughout the reaction period of 19.5 hours the wax is stirred vigorously. 100 grams of airoxidized wax having a saponification number of 232 is obtained. Additionally, 5.7 parts of a mixture, consisting essentially of formic, acetic and propionic acids, and 14.4 parts of a mixture of volatile aldehydes, esters and other oxygenated products, are also recovered as condensate from the exhaust air stream.

The 100 grams of air-oxidized wax is then placed in a stainless steel autoclave along with 500 grams of 70 percent nitric acid. A second pressure vessel is connected to the top of the autoclave in such fashion that the lower oxides of nitrogen given off during the reaction pass into a second vessel, where they are regenerated with air to higher oxides of nitrogen which are then condensed on a cooling coil in the vessel and returned by gravity through a line to the reaction vessel. The contents of the reaction vessel are maintained at about 90 C. for 30 hours. Throughout the reaction period the mixture is stirred and air is passed through the auxiliary vessel to regenerate the nitrogen oxides.

The oxidized mixture obtained at the end of the reaction period contains many compounds in two layers, one aqueous and one oily. The aqueous layer includes substantial amounts of dibasic carboxylic acids having an even number of carbon atoms, dibasic carboxylic acids having an odd number of carbon atoms and nitric acid. The layers are separated and to the aqueous layer is added 2 liters of nitropropane-l and the resulting mixture obtained is maintained at a temperature of about C. for about 8 hours until crystallization and precipitation of dibasic acids having an even number of carbon atoms is complete. The resulting mixture is then filtered and the precipitate obtained is washed with additional nitropropane-l to remove nitric acid and other soluble materials therefrom. The precipitate remaining consists essentially of dibasic carboxylic acids having an even number of carbon atoms, that is, from 4 to 10 carbon atoms.

The removal of dibasic carboxylic acids having an even number of carbon atoms from a mixture containing said acids is further shown below in Example II.

Example II The oxidized mixture obtained in Example I at the end of the -hour reaction period in the nitric acid reactor is distilled under a vacuum of 20 mm. of mercury at a maximum temperature of about 50 F. for a period of about 4 hours. At the end of the distillation period substantially all of the nitric acid has been removed and the mixture remaining comprises substantial amounts of dibasic acids having an even number of carbon atoms and dibasic acids having an odd number of carbon atoms. To this mixture is added 250 grams of nitropropane-l and the resulting mixture obtained is maintained at a temperature of about 32 F. for about 120 minutes until crystallization and preciptation of dibasic acids having an even number of carbon atoms is complete. The resulting mixture is then filtered and the precipitate obtained is washed with additional nitropropane-l to remove nitric acid and other soluble materials therefrom. The precipitate remaining consists essentially of dibasic carboxylic acids having an even number of carbon atoms, that is, from 4 to 10 carbon atoms.

The process of this invention is additionally illustrated below in Example III.

Example [[1 To a solution consisting of 6 grams of adipic acid and cc. of nitric acid havinga concentration of 70 percent is added 120 cc. of nitropropane-l. The mixture obtained is maintained at 32 F. for about 2 hours and crystals of dibasic acid are formed. The mixture resulting is filtered at a temperature of 32 F. and the dibasic acid crystals obtained are washed with 40 cc. of nitropropane-l at a temperature of about 32 F. The crystals are then washed at a temperature of about 75 F. with an additional 40 cc. of nitropropane-l maintained at 32 F. The yield is 5.74 grams of adipic acid, which corresponds to a recovery of 94 percent.

Separation of dibasic carboxylic acids having an even number of carbon atoms from a mixture containing the same, need not be carried .out in one stage but can be effected in a number of stages. This is illustrated below in Examples IV and V.

Example IV To a solution consisting of 1.55 grams of adipic acid and 1 0 c c. of nitric acid having a concentration of 70 percent is added 10 cc. of nitropropane-l. The mixture obtained is kept at a temperature of 32 F. for a period of about 30 minutes until crystallization of adipic acid has stopped. The resulting solution is then filtered at a temperature of about 75 F. and the crystals obtained are washed with 10 cc. of nitropropane-l at a temperature of about 75 F. The yield of adipic acid crystals is 0.99 gram. To the filtrate is added the 10 cc. of nitropropane-l used to wash the crystals, and the procedure of crystallization, filtration and washing with 10 cc. of nitropropane-l is repeated. An additional 0.27 gram of adipic acid is recovered. To the filtrate is added the 10 cc. of nitropropane-l used to wash the latter crystals, and the procedure of crystallization, filtration and washing with 10 cc. of nitropropane-l is repeated. An additional 0.30 gram of adipic acid crystals is recovered. Thus, a total of 1.56 grams of adipic acid is recovered, corresponding to a recovery of 100 percent (within the limits of experimental error).

Example V To a solution consisting of 5.99 grams of adipic acid and cc. of nitric acid having a concentrationof percent;is;added'240 c'c. of-nitropropane-l. The mixture obtained is kept .at a temperature of F. for a period of about 2 hours until crystallization has stopped. The resulting solution is then filtered at a temperature of about 75 F. and the crystals obtained are washed with 40 cc. of nitropropane-l at a temperature of 75 F. 'The yield of adipic acid is 3.72 grams. The filtrate plus the wash liquid, is held at a temperature of 32 -F for about 2 hours until crystallization had ceased. The resulting mixture is then filtered at a temperature of 32 F., resulting in a yield of 1.53 grams of adipic acid. The total yield of adipic acid, therefore, is 5.25 grams which corresponds to a recovery of 88 percent.

Employment of a nitrohydrocarbon, other than nitropropane-l, in accordance with the present process is shown below in Example VI.

Example VI To a solution consisting of 5.99 grams of adipic acid and 50 cc. of nitric acid having. a concentration of 70 percent is added 120 cc. of nitromethane. The mixture resulting is maintained at a temperature of 75 F. for a period of 2 hours until crystallization of adipic acid has stopped, after which the mixture is filtered. The crystals obtained are washed with 40 cc. of nitromethane, and the yield of adipic acid is 2.99 grams. The filtrate plus the wash liquid is then maintained at a temperature of 32 F. for about 2 hours until crystallization has ceased. The mixture obtained is filtered at a temperature of 32 F., resulting in a yield of 1.92 grams of adipic acid. The total yield of adipic acid obtained is 4.91 which corresponds to a recovery of 82 percent.

The employment of still another nitrohydrocarbon in accordance with the present process is illustrated below in Example VII.

Example VII To a solution consisting of 5.99 grams of adipic acid and 50 cc. of nitric acid having a concentration of 70 percent is added 120 cc. of nitrobenzenc. The mixture resulting is maintained at a temperature of 75 F. for a period of 2 hours until crystallization has ceased, after which the mixture is filtered. The crystals obtained are washed with 40 ice. of nitrobenzene and the yield of adipic acid is 2.64 grams. The filtrate plus the wash liquid is then maintained at a temperature of 32 F. for 2 hours until crystallization has ceased. The mixture obtained is filtered at a temperature of 32 F., resulting in a yield of 1.69 grams of adipic acid. The total yield of adipic acid is 4.33 grams which corresponds to a recovery of 72 percent.

The removal of other dibasic carboxylic acids having an even number of carbon atoms from a mixture containing such acids is shown below in Examples VIII (oxalic acid) IX (suberic acid) and X (sebacic acid).

Example VIII To a solution consisting of 6.00 grams ot' oxalic acid and 50 cc. of nitric acid having a concentration of 70 percent is added 120 cc. of nitropropane-l. The mixture resulting is maintained at a temperature of 75 F. for 2 hours until crystallization has ceased, after which the mixture is filtered at a temperature of 32 F. The crystals obtained are washed with 40 cc. of nitropropane-l, resulting in a yield of 4.98 grams of oxalic acid'wh ich corresponds to a recovery of 83 percent.

Example IX To a solution consisting of 6.00 g'ramslof suberic acid and 70 cc. of nitric acid having a concentration of 70 percent is added 120 cc. of nitropropane-l. The. mixture obtained is maintained at a temperature of about 32 F. for 2 hours until crystallization has ceased. The mixture obtained is filtered at a temperature of 32 F. The pre cipitate is washed with 40 cc. of nitropropane-l, resulting in 3.84 grams of suberic acid which corresponds to a re cove of 64 percent.

realm Example X A solution consisting of 6.00 grams of sebacic acid and 130 cc. of nitric acid is treated with 120 cc. of nitropropane-1 under conditions identical to those set forth above in Example IX. The yield of sebacic acid is 3.19 grams, which correspond to a recovery of 53 percent.

That dibasic carboxylic acids having an odd number of carbon atoms will not crystallize out of solution under the reaction conditions of the present invention and be recovered along with dibasic carboxylic acids having an even number of carbon atoms is shown below in Examples XI and XII wherein the dibasic acids present are malonic and azelaic, respectively.

Example XI To a solution consisting of 6.00 grams of malonic acid and cc. of nitric acid having a concentration of 70 percent is added 120 cc. of nitropropane-l. The resulting mixture is maintained at a temperature of 75 F. for one day. No appreciable precipitation occurs, and it is estimated that less than 0.1 gram of malonic acid crystallizes out of solution. The solution is then maintained at a temperature of 32 F. for one day, and again no appreciable precipitation occurs. It is estimated that the amount of malonic acid which crystallizes out of solution during the latter stage is less than about 0.1 gram.

Example XII To a solution consisting of 6.00 grams of azelaic acid and 40 grams of nitric acid having a concentration of 70 percent is added 120 cc. of nitropropane-l. The solution is allowed to stand at a temperature of 75 F. for 2 days but no visible precipitation occurs. The solution is then held at a temperature of about 32 F. overnight, and it is estimated that less than 0.1 gram of azelaic acid precipitates out of solution.

While the specific examples disclosed above relate to a process wherein a nitrohydrocarbon is employed to separate a dibasic carboxylic acid having an even number of carbon atoms from a solution containing said acid and either nitric acid or dibasic carboxylic acids having an odd number of carbon atoms or both, these examples are intended to be representative of the invention and not limiting. It is within the scope of our invention to remove dibasic carboxylic acids having an even number of carbon atoms from any mixtures or solutions wherein the remaining ingredients or parts thereof are substantially soluble in the nitrohydrocarbon.

Obviously, many modifications and variations of the invention, is hereinabove set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A method for removing at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms from a mixture containing at least one of said acids which comprises admixing with said mixture a nitrohydrocarbon,

, holding the resulting mixture for a time sufiicient to pre' cipitate said acid, and thereafter removing said precipitated acid from said resulting mixture.

2. A method for removing at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms of two to about 20 carbon atoms per molecule from a mixture containing at least one of said acids which comprises admixing with said mixture a nitrohydrocarbon, holding the resulting mixture for a time sufiicient to precipitate said acid, and thereafter removing said precipitated acid from said resulting mixture.

3. A method for removing at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms of two to about 20 carbon atoms per molecule from a mixture containing at least one of said acids which comprises admixing with said mixture a nitrohydrocarbon having from one to 6 carbon atoms per molecule,

7 holding the resulting mixture for a time suificient to precipitate said acid, and thereafter removing said precipitated acid from said resulting mixture.

4. A method for removing at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms of two to about carbon atoms per moleculefrom a mixture containing at least one of said acids which comprises admixing with said mixture at nitrohydrocarbon having from one to 3 carbon atoms per molecule, holding the resulting mixture for a time sufficient to precipitate said acid, and thereafter removing said precipitated acid from said resulting mixture.

5. A method for removing at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms of two to about l0 carbon atoms per molecule from a mixture containing at least one of said acids which comprises admixing with said mixture a nitrohydrocarbon having from one to 3 carbon atomsper molecule, holding the resulting mixture for a time sufiicient to precipitate said acid, and thereafter removing said precipitated acid from said resulting mixture. 7 V

6. A method for removing at least one aliphatic dibasic carboxylic acid having an even number or" carbon atoms of two to about 10 carbon atoms per molecule from a mixture containing atleast one of said acids which comprises admixing with said mixture a nitrohydrocarbon having from one to 3 carbon atoms per molecule in an amount sufiicient to dissolve substantially completely the remaining ingredients forming a part of the original mixture, holding the resulting mixture for a time sufiicient to precipitate said acid, and thereafter removing said precipitated acid from said resulting mixture.

7. A method for removing at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms of two to about 10 carbon atoms per molecule from a mixture comprising at least one of said acids and nitric acid which comprises admixing with said mixture a nitrohydrocarbon having from one to 3 carbon atoms per molecule in an amount sutficient to dissolve substantially completely said nitric acid, holding the resulting mixture for a time sufficient to precipitate said dibasic carboxylic acid, and thereafter removing said precipitated acid from said resulting mixture.

8. A method for removing at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms of two to about 10 carbon atoms per molecule from a mixture comprising at least one of said acids and at least one dibasic carboxylic acid having an odd number of carbon atoms Which comprises admixing with said mixture a nitrohydrocarbon having from one to 3 carhon atoms per molecule in an amount sulficient to dissolve substantially completely said latter dibasic carboxylic acid, holding the resulting mixture for a time sufficient to precipitate said dibasic carboxyiic acid having an even number of carbon atoms, and thereafter removing said precipitated acid from said resulting mixture.

9. A method for removing at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms of two to about 10 carbon atoms per molecule from a mixture comprising at least one of said acids, nitric acid and at least one dibasic carboxylic acid having an odd number of carbon atoms which comprises admixing with said mixture a nitrohydrocarbon having from one to 3 carbon atoms per molecule in an amount suificient to dissolve substantially completely said nitric acid and said latter dibasic carboxylic acid, holdingthe resulting mixture for a time suliicient to precipitate saiddibasic carboxylic acid having an even number of carbon atoms, and thereafter removing said precipitated acid from said resulting mixture.

10. A method for removing aliphatic dibasic carboxylic acids having an even number of carbon atoms from the product obtained as, a result of oxidation of parafiin hydrocarbgns to dibasic carboxylic acids which comprises admixingwith said product'a nitrohydrocarbon having fronron'e to 3 carbon atoms per molecule, in an amount to dissolve substantially completely the resulting product, holdin v theresulting product for a time suflicientto precipitate dib'asic carboxylic acids having an even number of carbon atoms in said resulting product, and thereafter removing said precipitated acids from said resulting product, 7

' 11. A method for removing at least one aliphatic dibasic carboxylic acid haying an even number of carbon atoms of two to about 10. carbon atoms per molecule from a mixture containing at least one of said acids which comprises admixing with said mixture nitromcthane in an amount sufilcient toidissolve substantially completely the remaining ingredients forming a part ofthe original mixture, holding the resulting mixture for a time sufficient to precipitate said, acid, and thereafter removing said precipitated acid frQ il Said resulting mixture.

12. A method for removing at least one aliphatic dibasic carboxylic acid-having an even number of carbon atoms of two to about 10 carbon atoms per molecule from a mixture; containing at least one of said acids which comprises admixing with said mixture nitropropane in an amount sufficient-to dissolve substantially completely the remaining ingredients forming a part of the original mixture, holdingthe. resulting mixture for a time; sutficient to precipitate said acid, and thereafter removing said precipitated acid from said resulting mixture.

13. A method for removing at least one aliphatic dibasic carb XY iG: acid having an even number of carbon atoms of twoto about 10 carbon atoms per molecule from a mixture containing at least one of said acids which comprises admixing with said mixture nitrobenzene in an amount ufilcient dis ol substan a ly o p y the remaining ingredients forming a part of the original mixture, holding the resulting mixture for a time suflicient to precipitate said acid, and thereafter removing said precipitated acid from said resulting mixture.

14. A method forremoving at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms of twoto about, 10 carbon atoms per molecule from a mixture comprising at least one of said acids and nitric acid which comprisesv admixing with said mixture nitromethane in an amount sufficient to dissolve substantially completely saidnitric acid, holding the resulting mixture for a time suflicientto precipitate said dibasic acid, and thereafter removing said precipitated acid from said resulting mixture.

15. A method for removing at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms of two to about 10 carbon atoms per molecule from a mixture comprisingat least one of said acids and nitric acid which comprises admixing with said mixture nitropropane in an amount sufificient to dissolve substantially completely said; nitric acid, holding the resulting mixture for a time sutlicient to precipitate said dibasic acid, and thereafter removing said precipitated acid from said resulting mixture. 7

16. A method for. removing at least one aliphatic dibasic carboxylic acid having an even number of carbon atoms oftwo, to about 10 carbon atoms per molecule from a mixture comprising at least one ofsaid acids and nitric acid which comprises admixing with said mixture nitrobenzene in an amount sufficient to dissolve substantially completely said nitricacid, holding the resulting mixture for a time. sufiicientlto precipitate said dibasic acid, and thereafter rern oying'said precipitated, acid from said resulting mixture. Y i

No r erences i 

1.A METHOD FOR REMOVING AT LEAST ONE ALIPHATIC DIBASIC CARBOXYLIC ACID HAIVNGAN EVEN NUMBER OF CARBON ATOMS FROM A MIXTURE CONTAINING AT LEAST ONE OF SAID ACIDS WHICH COMPRISES ADMIXING WITH SAID MIXTURE A NITROHYDROCARBON, HOLDING THE RESULTING MIXTURE FOR A TIME SUFFICIENT TO PRECIPITATE SAID ACID, AND THEREAFTER REMOVING SAID PRECIPITATED ACID FROM SAID RESULTING MIXTURE. 